A review on recent application of proton transfer photophysics of bipyridine-3,3′-diol in organized assemblies

The rich photophysics of the excited-state intramolecular proton transfer (ESIPT) process in 2,2′-Bipyridine-3,3′-diol (BP(OH) 2 ) system has interested a lot of researchers from both the experimental and the theoretical sides. The resemblance of BP(OH) 2 with Watson-Crick canonical base pairs of DNA has led to some interesting spectroscopic investigation. Several efforts have been made to arrive at a detailed mechanism of the ESIPT process that generates the “stepwise or concerted?” question, which has been a controversy for years. This controversy led to an enlightening discussion on excited state symmetry and symmetry conservation during the ESIPT process. Under this context, the time has ripened to make a concise view of the fundamental photophysics of this molecule in various organized media, like, micelles, vesicles, fibrils, etc. Moreover, the recent application of this molecule as a fluorescent marker is seen in many self-assembled aggregates formed by various biomolecules, which needs to be highlighted to let the researchers acknowledge it. With this aim, the review, however, by all means, is not an exhaustive article information collection that covers all aspects of ESIPPT, rather, it will mostly focus on the recent application of this probe in many organized assemblies. Graphical abstract